1. Field of the Invention
This invention is in the field of night vision devices which provide a visible image from low-level visible light or from light in the infrared (invisible) portion of the spectrum by use of an image intensifier tube. As used herein, the term "light" means electromagnetic radiation, regardless of whether or not this light is visible to the human eye.
Image intensifier tubes of such night vision devices generally include a photocathodes which is responsive to light in the red end of the visible spectrum as well as in the infrared spectral range to release photoelectrons.
Thus, the present invention is also in the field of such photocathodes.
The photoelectrons released by the photocathode within such an image intensifier tube may be amplified or multiplied by conventional devices such as a microchannel plate or dynode to provide, for example, a current indicative of a light flux, or to produce an image of a light source or of an object illuminated with infrared light.
One embodiment of a photocathode according to the present invention includes a fully-absorptive photon-absorbing layer of indium gallium arsenide (InGaAs), and an electron-emitting layer of indium potassium (InP).
2. Related Technology
Night vision devices which use an image intensifier tube are well known. Generally, such devices include an objective lens by which light from a distant scene is received and focused upon a photocathode of the image intensifier tube. A power supply of the device provides appropriate voltage levels to various connections of the image intensifier tube so that this tube responsively provides a visible image. An eyepiece lens of the device provides the visible image to a user of the device.
Particularly, the image intensifier tube includes a photocathode responsive to light photons within a certain band of wavelengths to liberate photoelectrons. Because the photons are focused on the photocathode in a pattern replicating an image of a scene, the photoelectrons are liberated from the photocathode in shower having a pattern replicating this image of the scene. Within the image intensifier tube, the photoelectrons are moved by an applied electrostatic field to a microchannel plate, which includes a great multitude of microchannels. Each of the microchannels is effectively a dynode, which liberates secondary emission electrons in response to photoelectrons liberated at the photocathode. The shower of secondary emission electrons from the microchannel plate are moved to a phosphorescent screen which provides a visible image in yellow-green phosphorescent light.
Conventional photocathodes are disclosed in each of the following United States or foreign patents:
U.S. Pat. No. 3,814,996, issued Jun. 4 1974, is believed to disclose a photocathode of an ternary alloy of indium, gallium, and arsenide of the formula Ih.sub.x Ga.sub.1-x As, in which "x" has a value of from 0.15 to 0.21.
U.S. Pat. No. 4,286,373, issued Sep. 1, 1981, is believed to disclose a photocathode of gallium arsenide at the photo-emitting layer, and is associated with a layer of gallium, aluminum, arsenide as a passivating layer.
U.S. Pat. No. 4,477,294, issued Oct. 16, 1984, is believed to relate to a photocathode of gallium arsenide as the photo-emitting layer, which is formed by hybrid epitaxy.
U.S. Pat. No. 4,498,225, issued Feb. 12, 1985, is thought to disclose a photocathode of gallium arsenide, formed on a glass substrate with intervening layers of gallium, aluminum, arsenide as passivation and anti-reflection layers.
U.S. Pat. No. 5,047,821, issued Sep. 10, 1991 is believed to relate to a transferelectron photodiode and photocathode structure in which a metallization at the electron-emitting face of the photocathode is supplemented by addition of a grid which is preferably of radial-spoke configuration. This photocathode includes a photon-absorbing layer which is only from 200 nm to 2 .mu.m thick. An electron-emitting layer of this photocathode is from 200 nm to 1 .mu.m thick.
U.S. Pat. No. 5,268,570, relates to a photocathode of indium gallium arsenide, grown on an aluminum indium arsenide window layer.
Similarly, U.S. Pat. No. 5,506,402, relates to a photocathode of indium gallium arsenide, grown on an aluminum gallium arsenide window layer.
British patent No. 1,478,453, issued Jun. 29 1977, is believed to disclose a photocathode comprising (Ga.sub.1-x Al.sub.x).sub.1-z In.sub.z As, wherein (0.ltoreq.z&lt;y).
It may be that none of these conventional photocathodes are capable of providing a desired level of spectral response in the 1 to 2 .mu.m wavelength band. Particularly, none of these conventional photocathodes are believe to be able to provide a sufficient response substantially at the 1.54 .mu.m wavelength which is provided by erbium-doped glass lasers. Use of such erbium-doped glass lasers is particularly desired for illumination, spotting, and designation uses because they are eye-safe. Further, conventional night vision equipment does not respond to light of this wavelength. That is, a photocathode having such a response is desired for night vision equipment in order to allow, for example, imaging using active illumination of a scene with such an erbium-doped glass laser. This would be a particular advantage in the military and police areas of imaging because present GEN-III night vision equipment is not able to provide detection of such laser light.
That is conventional S-20 (alkali-based) photocathodes will not provide an image to such light, and conventional semiconductor-based photocathodes, which generally employ GaAs, have a long-wavelength cutoff of about 900 nm (0.9 .mu.m). Accordingly, police equipped with advanced night vision equipment responsive to wavelengths above 1 .mu.m, and using 1.54 .mu.m laser illumination would be able to see in total darkness without providing an image to conventional GEN-III night vision equipment, and not allowing the users of such conventional equipment to sight on the illumination laser lights of the police.
The cutoff wavelength for a conventional semiconductor photocathode can be extended to the range of 900-1100 nm by using a ternary compound of indium, gallium, and arsenide. While the quantum efficiency of such photocathodes is less than conventional GaAs photocathodes, the greater photon availability under night-sight conditions compensates for this loss of efficiency. Further, the night sky is rich in light in the 1.1-1.8 .mu.m band. Attempts by researchers in the field to extend the spectral range for photocathodes deeper into the infrared portion of the spectrum have lead to the development of so called "transfer electron" photocathodes. These photocathodes are based on the transfer of thermalized electrons in the conduction band. These thermalized electrons are transferred to higher conduction bands under the influence of a reverse bias. In the higher conduction bands, the electrons can escape into the vacuum within an image intensifier tube. A coating of silver has been used on the electron emitting surface to provide a reverse bias and a Schottky barrier contact. These conventional photocathodes have shown some responses in the range from about 1.1 to about 1.6 .mu.m; but generally also needed to be cooled to temperatures considerably below room temperature in order to help their performance. That is, these photocathodes are believed not to have operated at room temperature while providing the desired response to 1-2 .mu.m light.
Further to the above, scientific uses of such a photocathode are many. For example, there exists now no acceptably inexpensive large-format photon detector for use in the 1-2 .mu.m range. Present photodiodes which are responsive in this wavelength band limit users to a tiny reception format (i.e., about 1-2 .mu.m diameter reception area) with no internal gain. The alternative prior to this invention was to use a high-cost photomultiplier tube which possesses a very limited lifetime, presents reliability concerns, may require cryogenic cooling, and has a high cost.
A large format photomultiplier tube able to provide a response in the 1-2 .mu.m range at room temperature would be desirable.